Pehur high-level canal, NWFP, Pakistan

This paper describes the design, construction and commissioning of a major irrigation project in Pakistan. It is the final stage of the Tarbela Dam project and follows two centuries of irrigation development in the Indus Valley. The Pehur high-level canal (PHLC) is now operational after a five-year construction period which began in 1997. The purpose of the project is to bring 30 m(3)/s of water from the Indus River at Tarbela Dam to the lower reaches of the existing Upper Swat canal (USC) system. This not only alleviates water shortage in the existing 100 000 ha Upper Swat scheme but will allow additional areas totalling about 25 000 ha to be brought under irrigation upstream of the confluence with PHLC. The combined system Tarbela-PHLC and Swat river-USC is a complex one that is aimed at optimising water use from the two different sources, which are in restricted supply at different times of the year. Accordingly the design philosophy of PHLC is different to all other major canals in Pakistan. The project included several novel principles of design, construction and operation, such as automatic downstream control, large parabolic canals and low-pressure pipelines for irrigation distribution. The main project comprises two tunnels totalling 6 km, two large inverted siphons, each over 3 m dia. and about 2 km long, 26 km of concrete-lined parabolic canal (the largest of its kind in the world) and reconstruction of the existing 45 km long Maira branch canal. Cross-regulation of main and branch canals is provided by 13 pairs of self-regulating float-operated gates. The rugged terrain necessitated about 40 cross-drainage structures in the form of culverts and super-passages. Also included was rehabilitation of the existing minor canal system covering 40 000 ha and the development of about 5000 ha of new irrigation based on a buried pipeline distribution network. Commissioning was delayed by difficult tunnelling conditions, and several design modifications were required in commissioning the canal control gates. The more unconventional facets of the project's design were initially met with scepticism by the engineering establishment, but have been mostly vindicated by the project's successful completion.